1. Field of the Invention
This invention relates in general to an adaptive depolarization-interference-compensator for digital radio relay systems utilizing multi-stage quadrature amplitude modulation (QAM) with common channel (Co-channel) operation of the orthogonally polarized signals which is not necessaily synchronous with the clock frequency and/or carrier frequency and uses transversal filters with devices for coupling signals out of the receiving branch of one polarization and to the input couple signals into the receiving branch of the other polarization.
So as to increase the bandwidth efficiency in digital radio relay links featuring multi-stage quadrature amplitude modulation (16QAM, 64QAM, . . . ) the possibility exists of using both polarization directions simultaneously in a channel.
2. Description of the Prior Art
The previous practice of using both polarization directions (vertical and horizontal) is limited to the orthogonally polarized transmission of adjacent channels so that by polarization decoupling to reduce the adjacent channel interference to an extent such that at least a partial overlap of neighboring spectra is possible. The use of both orthogonal polarization directions and common channel operation increases the bandwidth efficiency approximately by a factor of 2. However, the susceptibility to interference due to depolarization effects is substantially greater. Depolarization occurs, for example, during rain, snow and in the case of multi-path propagation. In the case of multi-path propagation, due to the antenna characteristics for the two polarization directions especially with deep dispersive fading, frequency selective depolarization occurs. The main effects of the depolarization are dependent upon propagation conditions which are naturally time variable.
For compensation of these depolarization effects, so-called cancellers have been provided which consist of adaptive phase shifters arranged crosswise with an automatic gain control AGC between the two receiving channels. This serves to reduce the coupling of the two channels caused by depolarization. Such arrangement can be constructed in the RF range, the IF range or in base-band. The effectiveness in the case of frequency selective depolarization is only limited, however. An article entitled "A Decision Directed Network For Dual Depolarization Cross-Talk Cancellation" by William J. Weber, published in ICC 79 Pages 40.4 to 40.4.7 describes an arrangement of this kind.
An arrangement of this kind is also disclosed in U.S. Pat. No. 3,735,266 which describes a method and arrangement for reducing the cross-talk in microwave transmission systems in which the information is transmitted in two polarizations or channels. In this system, a pilot signal is additionally transmitted in each channel and the components of which are detected in corresponding receiver devices. Signals are thus derived so as to eliminate cross-talk.
For improved compensation of the frequency selective depolarization, transversal filters are used as cancellers either in the IF plane or in the base-band. These consist of so-called Baud-spaced transversal filters, in other words, the delay time between two consecutive tappings is equal to one symbol period. However, such cancellers are not particularly suitable when the clock signals of the data flows of the two polarization directions are not synchronous.
The setting of the transversal filters is fundamentally accomplished using two algorithms, one being the zero forcing algorithm and the other the minimum-mean-square-error algorithm (MMSE). Both algorithms operate using samples of the multi-level base-band signals. It is thus clear that the setting of the filters is dependent upon the clock phase. So as to adjust the canceller, it is necessary to use the clock rate of the interference signal so as to achieve a stable setting of the coefficients. The zero forcing algorithm causes a precise identify between the compensation signal and the interference signal at a number of sampling instants which correspond to the filter length (number of coefficients). The MMSE algorithm results in a minimum mean square error between the compensation signal and the interference signal at all sampling times. In the case of asynchronous clock signals, however, the sampling time of the interference signal generally does not conform with the sampling time of the desired signal. Thus, in case of a Baud-spaced transversal filter, the sample values in the symbol clock spacing do not satisfy the sampling theorem using a realizable Nyquist pulse shaping. Thus, conformity between the compensation signal and the interference signal is not very good outside of the sampling times used for the acquisition of the coefficient setting. See, for example, FIGS. 8A and 8B which for a simple base-band model with selective depolarization interference represents the interference signal and the compensation signal in FIG. 8A and represents the residual interference for a canceller composed of a 7Tap-Baud-spaced transversal filter in FIG. 8B. In this system, the canceller is set according to the zero forcing algorithm as can be seen from the zero positions of the residual interference according to the filter length. Between the sampling times, the residual interference is relatively great.